δ13C provides a robust indicator of the sources of suspended sediment in a tropical river traversing forested and agricultural land

Degradation of freshwater and marine ecosystems by sediment and associated pollutants is widespread, We set out to determine the sources of suspended sediment, using composite fingerprinting, in the Tully River, which discharges into the Great Barrier Reef lagoon. Samples of suspended sediment combined over a whole wet season were taken from the Tully River and two of its main tributaries, Samples of potential source material were taken from 102 sites covering several land use and geological categories. When all 23 measured properties (mostly total elemental contents) were included in the fingerprint, 50% of the suspended sediment in the Tully River was attributed to sugarcane surface soil, 15% to other land uses, and 35% to channels, which are all in sugarcane growing areas on Quaternary alluvium and colluvium. However, mean relative errors were quite high. When mineral properties were excluded from the fingerprint, land use sources could be discriminated with reduced mean relative errors, δ13C separated forest versus sugarcane, and 613C in combination with C:N ratio separated surface soil versus channels. Fingerprints based on organic properties attributed >60% of suspended sediment to channel erosion. The results show that caution is needed when applying and interpreting the composite fingerprinting approach in some environments

Storage and release of fossil organic carbon related to weathering of sedimentary rocks

International audienceThe biogeochemical carbon cycle, which plays an undeniable role in global climate change, is defined both by the size of carbon reservoirs (such as the atmosphere, biomass, soil and bedrock) and the exchange between them of various mineral and organic carbon forms. Among these carbon forms, fossil organic carbon (FOC) (i.e., the ancient organic matter stored in sedimentary rocks) is widely observed in modern environments but is not included in the supergene carbon budget. Using a digitized map of the world and an existing model of CO2 consumption associated with rock weathering, we establish the global distribution of FOC stored in the first meter of sedimentary rocks and a first estimation of annual FOC delivery to the modern environment resulting from chemical weathering of these rocks. Results are given for the world's 40 major river basins and extended to the entire continental surface. With a mean value of 1100 109 t, mainly controlled by shale distribution, the global FOC stock is significant and comparable to that of soil organic carbon (1500 109 t). The annual chemical delivery of FOC, estimated at 43 106 t yr− 1 and controlled by the areal distribution of shales and runoff, is of the same order of magnitude as the FOC output flux to oceans. Chemical weathering of bedrock within the Amazon basin produces one-quarter of the total global flux of FOC derived from chemical weathering, and thus is expected to govern FOC release on a global scale. These results raise important questions concerning the role of FOC in the modern carbon cycle as well as the origin and the budget of carbon in soils and river

High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician

It has been hypothesized that predecessors of today’s bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO2 concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km3 rock) yr−1, defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than today’s global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO2 concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate